I know that if I sell a stock and realize a large cap gain, I will have to pay taxes on that gain because none were withheld. I also know that if you end up owing more than $1000 in taxes when you file, you will get penalized. My question is this -- when is the cutoff for realing a cap gain, and not paying estimated taxes? To illustrate -- I sell a stock on Dec 15 99 and realize a long term cap gain of $100,000. I will owe $20,000 of this to the IRS, so when I file my taxes in Apr of 2000, assuming all my other taxes issues net to 0, I will owe the IRS $20,000. Will I get penalized for this? I wouldn't think so. The other scenario is that I sell the stock on Jan 1, 99. Again, $20,000 of long term cap gains, but this time I held onto the gains for over a year. Please help me.